Gulf storm strengthening but disorganized... impacts beginning along coast

Rainfall from Potential Tropical Cyclone 16 is reaching Louisiana over to the northern Florida peninsula as of Friday morning.  This will continue and become more widespread, and storm surge will also begin to increase for places to the right of the storm's track (with onshore wind).  Although it narrowly lacks the organization to be classified as a tropical or subtropical cyclone and earn a name, it now has peak sustained winds of 60 mph.  If it does get named, it would be Nestor.

As anticipated, it is looking less tropical and more subtropical with time, which doesn't really change the impacts, just the formal classification if it ever gets that far.  The center is still expected to reach the coastline near Panama City early Saturday morning with tropical storm force winds.  This is the same area where Category 5 Hurricane Michael made landfall last October and the long road to recovery has just begun.

Looking at the winds, the map below shows the probability of an area experiencing tropical storm strength winds over the coming three days along with the most likely arrival time of those winds.  Note that the whole southeast U.S. coast from central Florida up to North Carolina should be preparing for some potentially damaging winds (mostly damaging to trees). 

Perhaps the biggest hazard posed by this will be storm surge, and just because of where it's headed. The Big Bend area of Florida is shaped perfectly to allow winds to bulldoze water onshore.  The rainfall could be locally problematic, but overall, the storm's relatively quick forward speed will limit the amount of rain that will fall.  As of Friday morning, it's moving toward the northeast at 22mph and that is expected to increase with time.  And to round out the big 4 hazards, there is an elevated tornado threat to the right of the forecast track.  The NWS Hurricane Threats & Impacts (HTI) graphics as of Friday morning are shown below:

If you missed this on Twitter yesterday, it's worth pointing out that this system has been "on the radar" since last Thursday, just oozing its way up from central America and very slowly trying to get organized. And even as of today, it still has not developed, but is very close.
https://twitter.com/BMcNoldy/status/1184879258071494657

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Gulf storm will bring rain, storm surge to northern Gulf coast on Fri-Sat

The disturbance that has been a feature of interest for one full week now is finally taking shape in the western Gulf of Mexico.  Although it is not forecast to become very strong, it will bring the threats of flooding rain and storm surge to areas from Louisiana to the northern Florida peninsula in the coming days.

Identified as "Invest 96L" for now, it could get transitioned to Potential Tropical Cyclone 16 before it forms which would facilitate tropical storm and storm surge warnings to be issued if warranted. If it does become a tropical or subtropical cyclone, the next number on deck is 16 and the next name is Nestor.

This 3-day rainfall forecast is valid through Sunday morning and highlights areas that could see some heavy rain:

Three-day rainfall forecast, from Thursday morning through Sunday morning. (NOAA/WPC)

The latest 50-member European model ensemble has some members with trackable low pressures, and they are all with wind speeds in the depression-to-storm range... and all other model guidance agrees with this type of track and limited intensity.  Although impacts will begin prior to landfall, landfall should occur early Saturday in the area between the mouth of the Mississippi River (LA) over to the Big Bend area (FL).

This intensity, track, and month, all closely resemble Tropical Storm Josephine in 1996.  It formed in the west-central Gulf, tracked toward the northeast, and made landfall in Florida's Big Bend area which is especially sensitive to storm surge.  Below is a newspaper clipping I have from October 9, 1996; it documents a 5-7 foot storm surge with no wind damage near Steinhatchee (on the coast, west of Gainesville). As a resident said, "I guess it don't have to be too bad to get bad. Things like this happen around here."

Newspaper from October 9, 1996 regarding Tropical Storm Josephine. (Reading Eagle)

Elsewhere in the basin, things are quiet, so all eyes are watching Invest 96L in the Gulf.

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Odds increasing of Gulf storm this weekend

Surface wind streamlines as of Wednesday morning. Two circulation centers are apparent: one on each side of the Isthmus of Tehuantepec. (earth.nullschool.net)

Since yesterday's update, the disturbance over central America has drifted into the southern Bay of Campeche.  It's still disorganized, and is really close to another disturbance on the Pacific side of the low-lying 130-mile-wide Isthmus of Tehuantepec!  This has been a feature of interest going back to last *Thursday* when it was a disorganized blob off the east coast of Nicaragua.  These things can take a long time to cook (remember watching Michael for about a week last year before it became a Depression?).  While it could end up near where Michael made landfall last October, there are zero indications that this will intensify much.

The most recent European model ensemble indicates a pretty decent chance of this developing in the coming days as it heads north to northeast. Some of the ensemble members have a trackable low pressure center near the northern Gulf coast on Saturday, but none are strong (shown here).  If this becomes a tropical storm, the next name on the list is Nestor.  Nestor was introduced to this list in 2013 after Noel '07 was retired.

As I shared yesterday, there's a short list of seven named storms that developed in the western Gulf of Mexico after October 1 since 1960.  The one that took off toward the northeast into Florida's Big Bend region was Josephine in 1996.

Tropical Depression 15, the one that was way out by Cabo Verde, has dissipated and never reached tropical storm status and so was never named.

As of today, the Atlantic has had 13 named storms, 5 of which became hurricanes, and 3 of those became major hurricanes (Category 3+).  I whipped up a seasonal tracking map through today to refresh your memories on when and where things happened.

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TD 15 forms by African coast, also closely watching Gulf of Mexico

Enhanced satellite image of Tropical Depression 15. (EUMETSAT)

On Monday afternoon, a very strong and late African wave developed into Tropical Depression 15, way out at 20.2°W -- just 200 miles offshore! The Cabo Verde season, in which tropical waves exit the African coast near the Cabo Verde islands, typically spans mid-August through mid-October... but by mid-October it's pretty meager. As Phil Klotzbach at CSU pointed out, this is the easternmost tropical cyclogenesis this late in the year on record.

The forecast is not very threatening to anyone though. It may never reach tropical storm status as it passes by the Cabo Verde islands then off toward the northwest where it is forecast to dissipate.

Elsewhere, a disturbance over central America has some potential for development in the Gulf of Mexico later this week.  The National Hurricane Center is giving it a 40% chance of development into at least a depression by the end of the weekend.

Global model ensembles are divided on the system developing on the west side of Mexico or the east, but the trend has been shifting eastward, favoring the Gulf.  At this point, model guidance does not suggest that it becomes anything too strong, but that can change at this long lead time. So we will just keep a very close eye on it and model trends... especially interests in the western Gulf this weekend.  The next name on the list is Nestor.

Looking back to the start of the satellite era in 1960, I could find just seven named storms that developed in the western Gulf after October 1.  The two most recent were Marco (2008) and Matthew (2004). Jerry (1989) is the one that made landfall near Galveston as a Category 1 hurricane on October 16.

Tracks of seven named storms that formed in the western Gulf after October 1 (1960-2018).

Today is also the 14th anniversary of Hurricane Wilma's formation (Tropical Depression 24 at the time).  If you missed it on Twitter, there's a thread with new and interesting graphics at https://twitter.com/BMcNoldy/status/1184106020299104261

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Melissa forms off northeast U.S. coast

Tropical Storm Melissa, the season's 13th named storm, began its journey over North Carolina on Monday, then left the coast on Tuesday and continued to slowly organize... it was finally upgraded to a subtropical storm on Friday morning.  However, it may only be around for another day or so. Melissa will be yet another short-lived and messy storm this year, similar to Andrea, Chantal, Erin, Fernand, and Imelda -- those five storms combined were named for less than four days!

Peak sustained winds are at 50 mph and further weakening is expected as it accelerates toward the east into higher wind shear and colder water.  There are no associated tropical storm or storm surge watches/warnings. The storm is very tiny; the tropical storm force winds extend an average of just 45 miles from the center!

The recent transition from a subtropical cyclone to a tropical cyclone is nicely illustrated in this cyclone phase space diagram. It's a bit technical, but the low starts at location A on Wednesday, it's presently at location C on Saturday, and the forecast is at location Z on Friday. In this model-based analysis of the storm's structure, we see that it began as an asymmetric warm-core system (subtropical), it migrated into the symmetric warm core realm (tropical), is currently in a near-neutral zone, and is headed for the asymmetric cold core realm (extratropical).  So, while these classifications have a bit of subjectivity to them, there is also an objective foundation on which the transition is based.

Cyclone phase space diagram for Tropical Storm Melissa. (FSU)

Elsewhere, the Atlantic basin should be quiet for the foreseeable future. But don't tune out just yet... even late October has a history of producing some infamous storms!

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Lorenzo's trip to Ireland

Large and powerful Lorenzo has just passed the Azores and is headed toward Ireland.  It still has peak sustained winds of 80 mph and is racing toward the northeast at 43 mph. Tropical storm force winds extend as far as 390 miles from the center, while hurricane force winds extend as far as 150 miles from the center.

It will undoubtedly be a major problem in Ireland, with destructive winds and significant storm surge.  Port cities such as Galway will experience severe flooding on Thursday. Tropical storm force winds should arrive in Ireland on Thursday morning, followed quickly by hurricane force winds and a rapidly rising storm surge.  I have a long radar loop from Ireland at http://bmcnoldy.rsmas.miami.edu/tropics/radar/.

I found 18 tropical or post-tropical cyclones that passed within 100 miles of the Irish coast since 1851, and the most recent was Ophelia in October 2017, which made landfall as a post-tropical cyclone with sustained hurricane-force winds (80mph).  Debbie was the same intensity and was debateably still a hurricane when it made landfall in northwest Ireland in September 1961.  Edna was a little stronger than Ophelia and Debbie (85mph), and made landfall in northwest Ireland in September 1953 as a post-tropical cyclone.  Then the strongest on record was a post-tropical cyclone with nearly Category 2 sustained winds (90-95mph) that made landfall in southwest Ireland in September 1883.  We will see on Thursday where Lorenzo will fit into history.

Tracks of tropical and post-tropical cyclones that passed within 100 miles of Ireland since 1851. The gray lines indicate the storm was post-tropical.

Lorenzo formed back on September 23, and it alone has generated 27% of the season's total ACE (Accumulated Cyclone Energy) so far.  With this boost, the ACE is at 145% of average for this date, using the past 50 years as a climatology.

Elsewhere, the Atlantic basin remains relatively quiet for the foreseeable future.  There is a disturbance south of Cuba that has a low chance of development as it heads west toward the Yucatan Peninsula, and there's a wave still over Africa that models indicate could develop early next week in the far eastern Atlantic.

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